It is common for liquids in a chemical process plant to be held in and pass through a tank or series of tanks between processing operations. Such tanks may be called (among other names) storage tanks, buffer tanks, intermediate tanks or capacity tanks. They may be operated continuously or discontinuously (batchwise). In continuous operation, the liquid is substantially continuously supplied to and substantially continuously extracted from the tank. It is common for continuously-operated tanks to be equipped with an instrumental level measurement and control system, so that the volume or the residence time of the liquid stored in the tank can be controlled. The instrumental system may be designed so that normal variations in demand cause a signal to be sent to earlier stages in the process where the signal acts to produce corresponding variations in supply. This is known as feedback control. Continuously-operated storage tanks can also be equipped with control systems which respond to more extreme variations, for example temporary cessation of supply or demand, and therefore act as buffer or surge tanks. If supply is temporarily interrupted, then demand can be fulfilled for a limited time from the volume of liquid stored in the tank. If demand is temporarily interrupted, then supply can continue at the same or a reduced rate using the excess capacity of the tank for storage until demand is restored.
Certain liquids have time-dependent properties, and it is desirable to control their residence time in a process plant on the microscopic as well as the macroscopic scale. Examples of such liquids include polymer dopes for the manufacture of regenerated cellulose articles, for example fibres and films. Examples of this type of liquid include solutions of cellulose in tertiary amine N-oxides, for example, N-methylmorpholine N-oxide, which are useful in the manufacture of regenerated cellulose articles such as fibres and films. It is desirable to minimise the residence time of these solutions in a manufacturing plant, and to ensure uniform and consistent flow so that stagnation is avoided. Another example of such a liquid is viscose. This is prepared by dissolving sodium cellulose xanthate in aqueous sodium hydroxide; filtered; deaerated; and extruded through a suitable die to form regenerated cellulose fibre or film. Freshly-prepared (young) viscose is referred to as being "unripe", and must be stored for a controlled time while chemical changes take place in it to render it suitable for extrusion ("ripe"). Old viscose is referred to as being "overripe", and it too is unsuitable for extrusion. Furthermore, on extended storage viscose sets solid and becomes useless, and this can cause damage to the process plant itself. Finally, it is known that ripe viscose has superior properties to a mixture of overripe and underripe viscoses having the same degree of ripeness.
Storage tanks of the type hereinabove described are not entirely satisfactory as continuously-operated tanks for the storage of liquids which have time-dependent properties, since they tend to give regions of non-uniform and discontinuous flow and to have zones of stagnation. It is conventional to equip such tanks with a suitable mechanical stirrer to mix the liquid stored in the tank. Use of a stirrer is especially necessary with high viscosity liquids, for example viscose and solutions of cellulose in tertiary amine N-oxides, since such liquids follow a laminar flow regime and are therefore particularly prone to stagnation. In contrast, low viscosity liquids, for example water, generally follow a turbulent flow regime in which stagnation is less likely to occur. Use of such a stirrer involves additional expenditure for the installation of the stirrer itself and on the power required to operate it. Furthermore, the stirrer mixes together young and old liquid contained within the tank, so that residence time on the microscopic scale is not controllable as accurately as could be desired. The flow of the liquid through such a stirred tank does not approximate to a "first in, first out" principle.
GB-A-841,403 says that where a supply device continuously supplies a viscous material (for example, margarine) and an accepting device intermittently accepts a quantity of the material, that it is desirable to interpose a compensating device which compensates for the difference in the rates of the supply and acceptance. It describes a compensating device which comprises a chamber having an outlet, a piston movable within the chamber so as to vary its capacity and adapted to be urged to decrease the capacity of the chamber, and inlet means for conducting viscous material under pressure to the face of the piston. At least part of the inlet means is situated in the chamber between the face of the piston and the outlet, and is movable with the piston. The piston may operate a valve in the supply line by means of a mechanical linkage, whereby a reduction in the capacity of the chamber acts to open the valve and an increase in the capacity of the chamber acts to close the valve. In use, the piston intermittently operates to expel aliquots of the viscous material from the chamber to the accepting device, so that for example margarine can be packaged in small tubs. GB-A-841,403 only describes a process in which the accepting device accepts the material intermittently, and does not mention any process in which both supply and acceptance are continuous.
DE-A-3,416,899 describes a machine for decorating cakes, for example with lettering in chocolate. A batch of chocolate contained in a cylinder is expelled through nozzles onto the cakes to be decorated by the operation of a pneumatically-driven piston acting within the cylinder. Movement of the piston to a lower position on expulsion of the chocolate activates a low-level limit switch. Activation of this switch causes signals to be sent which suspend the operation of the piston and which start a gear pump. The gear pump pumps chocolate from an external hopper through a flexible tube connected to the interior of the hollow piston rod and thence into the cylinder. This pumping causes the piston to move to an upper position where it activates a high-level limit switch indicating that the cylinder has been filled with a fresh batch of chocolate. Activation of this switch causes signals to be sent which stop the gear pump and permit the expelling operation of the piston to be resumed. The cycle is repeated as necessary. DE-A-3,416,899 does not mention any process in which either the supply of chocolate to or the discharge of chocolate from the cylinder is continuous.